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Investment Securities - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013 Securities	Sep. 30, 2012	Sep. 30, 2013 Securities	Sep. 30, 2012	Dec. 31, 2012 Securities
Investment Securities [Line Items]
Number of securities in an unrealized loss position	171		171		57
Percentage of combined depreciation of the combined reported value of the aggregate securities portfolio	4.43%		4.43%		2.78%
Carrying value of securities pledged	$ 279,880,000		$ 279,880,000		$ 292,880,000
OTTI charges incurred	0	942,000	0	942,000
Reduced severity rate	15.00%		15.00%
Severity rate reduction period	6 years		6 years
Mortgage-backed securities Non-Agency Alt-A residential
Investment Securities [Line Items]
Voluntary constant prepayment rate	9.70%		9.70%
Constant default rate scenario	12.00%		12.00%
Remaining underlying mortgages default period	3 years		3 years
Severity rate	66.00%		66.00%
Equity Securities
Investment Securities [Line Items]
OTTI charges incurred	$ 0	$ 0	$ 0	$ 0